In the Worldwide Interoperability for Microwave Access (WiMAX) Forum, network reference architectures based on the IEEE802.16e broadband wireless access technology are defined. The WiMAX standard defines beside an access service network (ASN), a connectivity service network (CSN). The access service network comprises, for example, a base station (BS), in particular a radio base station and an access service network gateway (ASN-GW). The connectivity service network provides services like authentication, authorisation and accounting (AAA), dynamic host configuration protocol (DHCP) and a mobile internet protocol home agent (MIP HA). The home agent usually also constitutes an internet protocol router. Via the connectivity service network a terminal, for example, a mobile station (MS) or a subscriber station (SS) may have an access to the internet or to services which are specific for the respective operator, like, for example, an internet protocol multimedia subsystem (IMS).
A WiMAX roaming architecture comprises, for example, an access service network, a visited connectivity service network (V-CSN) and a home connectivity service network (H-CSN). In case of roaming, the access service network is connected to the visited connectivity service network, but not to the home connectivity service network. However, usually in the home connectivity service network the user specific data is managed, but not in the visited connectivity service network. Therefore, if a terminal or a mobile station accesses to an access service network, the mobile station will be authenticated by an authentication, authorisation and accounting server of the home connectivity service network, the home authentication, authorisation and accounting (H-AAA) server. In this case the authentication, authorisation and accounting server in the visited connectivity service network, the visited AAA (V-AAA) server only serves as an AAA proxy server. WiMAX uses for authentication an extensible authentication protocol (EAP) which may be transported within the AAA protocol corresponding to RADIUS OR DIAMETER. According to the WiMAX standard, the user data will be transported between the access service network and the connectivity service network in a mobile internet protocol (MIP) tunnel. The MIP home agent will be localised in the connectivity service network. During the MIP registration an authentication of the subscriber with the home AAA server takes place, which home AAA server also provides the key or key material to the home agent during authentication. The protocol between the home agent and the home AAA server is, again, RADIUS OR DIAMETER. In case of roaming, the WiMAX standard defines that the home agent is provided either in the home connectivity service network or in the visited connectivity service network. In case the home agent is provided in the home connectivity service network, the home agent address may be provided from the home AAA server to the access service network during the EAP authentication phase. In case the home agent is provided in the visited connectivity service network, the visited AAA server inserts the local home agent address in the AAA signalling to the access service network, owing to the provision of the home agent in the visited connectivity service network.